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Asset Retirement Obligations (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Dec. 31, 2013
Changes in ARO liability          
Asset retirement obligation, beginning of period     $ 3,029 $ 2,145  
Additional liability incurred     567 162  
Liabilities acquired     3,678 471  
Liabilities settled     (10) (14)  
Accretion expense 127 46 303 134  
Revisions in estimated liability     588 0  
Asset retirement obligation, end of period 8,155 2,898 8,155 2,898  
Less current portion 40 20 40 20  
Asset retirement obligations - long-term $ 8,115 $ 2,878 $ 8,115 $ 2,878 $ 2,989